Create refocus effect for a image?
Ok I got it here. It's hard to do this in only blender. Using photoshop would simplify this..
    The idea is simple. Open photoshop (or GIMP whatever..) => load the image in => Cut it into two (Each one has a focal point, near and further) => Export them as PNG => Load the two images into Blender => Composite them, blur each one at a time to fake this defocusing effect!